A lithium ion secondary battery is a storage battery having high energy density and is therefore used as a main power source of various portable appliances. A chemical battery such as a lithium ion secondary battery includes, in general, a positive electrode, a negative electrode and a separator placed between the positive electrode and the negative electrode. This separator has a role of keeping an electrolyte while electrically insulating the positive electrode and the negative electrode.
Currently, in a lithium ion secondary battery, an electrode group having a positive electrode and a negative electrode that are wound with a separator interposed therebetween is mainly used. In general, a positive electrode used in such an electrode group has a positive electrode current collector such as an aluminum foil and the like and a positive electrode mixture layer including lithium composite oxide and the like carried thereon. Likewise, a negative electrode has a negative electrode current collector such as a copper foil and the like and a negative electrode mixture layer including graphite and the like carried thereon. As the separator, a microporous thin membrane sheet and the like composed of a resin such as polyethylene and the like are used.
When the above-mentioned sheet-shaped separator composed of a resin is melted or shrinks by overheating, internal short circuit is generated. In this case, because of short circuit reaction heat generated instantly, a separator further shrinks to enlarge a short circuit part, and further enormous reaction heat is generated. Thus, there is a problem of promotion of abnormal overheating.
Recently, for avoiding such a problem, there is a suggestion on formation of a porous heat-resistant layer on the surface of a positive electrode mixture layer and a negative electrode mixture layer (see, Japanese Laid-Open Patent Publication (JP-A) No. 7-220759). The porous heat-resistant layer is formed by applying a slurry containing fine particles and a binder on the surface of a mixture layer and drying the slurry.
The short circuit reaction is known to be most vigorous when a positive electrode current collector comes into contact with a negative electrode current collector and a negative electrode mixture layer. Therefore, there is a suggestion on formation of a porous heat-resistant layer only on a portion at which internal short circuit tends to occur. Specifically, there is a suggestion on formation of a porous heat-resistant layer composed of a powder and a binder resin, on an exposed part of a positive electrode current collector and a negative electrode current collector provided for welding of a lead current collector (see, JP-A No. 2004-63343). The above-mentioned powder has a heat-resistance of 500° C. or more.
By the technology described in JP-A No. 7-220759, it is possible to realize a lithium ion secondary battery excellent in short circuit-resistance. However, even if the above-mentioned porous heat-resistant layer has ion conductivity, its ion conductivity is small, and hence, the ion conductivity of an electrode plate lowers, to increase reaction resistance.
Therefore, if such a porous heat-resistant layer is provided on the whole region in which an electrode reaction is performed, a charging and discharging reaction does not occur easily.
As described above, JP-A No. 2004-63343 describes formation of a porous heat-resistant layer only on a portion at which internal short circuit tends to occur due to overheating. However, currently, it has been known that the probability of occurrence of internal short circuit on a portion as described in JP-A No. 2004-63343 is not so high.
Then, an object of the present invention is to provide a lithium ion secondary battery capable of securing a battery reaction so as to maintain battery characteristics while efficiently preventing internal short circuit by overheating.